This disclosure generally relates to piezoelectric actuators, and more particularly to a compact hydraulic compensator for a piezoelectric actuator of a fuel injector for an internal combustion engine.
A conventional piezoelectric actuator can include a ceramic structure that changes a dimension when an electric potential is applied across the structure. Typically, the dimension can change, for example, approximately 0.12%. The dimension change for an actuator having a plurality of individual structures stacked along an axis is multiplied as a function of the number of structures in the piezoelectric actuator stack. A voltage application can result in a nearly instantaneous expansion of the actuator and corresponding movement of any structure connected to the actuator. In the field of automotive technology, especially, in internal combustion engines, it is believed that there is a need for the precise opening and closing of an injector valve element for optimizing the spray and combustion of fuel. Therefore, in internal combustion engines, piezoelectric actuators are now employed for the precise opening and closing of the injector valve element.
During operation, the components of an internal combustion engine can experience significant thermal fluctuations that result in the thermal expansion or contraction of the engine components. It is believed that, in a fuel injector assembly, the valve body may expand during operation due to the heat generated by the engine and a valve element may contract due to contact with the relatively cold fuel. If a piezoelectric actuator stack is used for the opening and closing of an injector valve element, the thermal fluctuations can result in valve element movements that can be characterized as an insufficient opening stroke, or an insufficient sealing stroke. This is because of the low thermal expansion characteristics of the piezoelectric actuator as compared to the thermal expansion characteristics of other engine components. For example, if a piezoelectric actuator stack is capable of 30 microns of movement and a valve element contracts 10 microns due to temperature fluctuations, the piezoelectric actuator stack has lost 33% of its overall movement. Therefore, any expansions or contractions of a valve element can have a significant effect on the fuel injector operation.
It is believed that a variety of component materials have been evaluated in order to identify a combination that has substantially similar thermal expansion properties throughout the range of operating conditions to which a fuel injector is exposed. Generally, the component materials that have been evaluated either do not exhibit sufficiently similar thermal expansion properties or involve exotic materials that are expensive or difficult to manufacture.
It is believed that there are a number of disadvantages in attempting to match the thermal expansion properties of different components. These disadvantages are believed to include merely approximating a change in length of the piezoelectric actuator stack, or accurately approximating the change in length of the piezoelectric actuator stack for a narrow range of temperature changes.
A hydraulic bearing can also provide compensation for a fuel injector. Referring to FIG. 4, an example of a conventional hydraulic bearing 100 includes a single cylindrical body 102 and a pair of chambers 104a, 104b located along a longitudinal axis 106 of the bearing 100. The chambers 104a, 104b are separate by a portion of the body 102 that includes a modified screw orifice 108. A first piston 110 that contiguously engages an end of a piezoelectric device 112 also defines a portion of the chamber 104a. A second piston 114 also defines a portion of the chamber 104b. The second piston 114 includes a plug 114a that is used in a hydraulic oil filing/purging operation. In the illustrated example, the plug 114a also centers a compression spring 116. An adjusting screw (not shown) installed in a cap portion of a fuel injector housing (not shown) varies the compression force provided by the spring 116. A flange 102a fixes the body 102 with respect to the fuel injector housing. The hydraulic bearing 100 controls or damps movement of the piezoelectric device 112 by virtue of the force required to displace fluid through the orifice 108. The size of the orifice 108 determines the damping effect of the hydraulic bearing 100. As the hydraulic bearing 100 experiences expansion or compression, e.g., due to thermal changes, the pistons 110,114 move, thereby displacing the fluid through the orifice 108. However, the fluid being forced through the orifice 108 resists rapid movement of the pistons 110,114. By reducing the size of the orifice 108, stiffer compensation is provided by the hydraulic bearing 100.
Conventional hydraulic bearings are believed to suffer from a number of disadvantages. These disadvantages are believed to include an elongated longitudinal dimension that adds to the overall length of a fuel injector, and a great number of precision components that are expensive to manufacture and assemble.
Thus, it is believed that there is a need for a compact, low cost, and accurate device to compensate for the changes in operation as a fuel injector experiences dimensional changes, e.g., due to temperature fluctuations.
The present invention provides a compensator for longitudinally positioning along an axis a device relative to a body. The compensator comprises a first tube, a second tube, a piston, and fluid. The first tube extends along the axis from a first end portion that occludes the first tube. The second tube is telescopically received in the first tube. The second tube extends along the axis from a second end portion that generally occludes the second tube and that defines an orifice. The piston is telescopically received in the second tube. And the fluid is displaceable through the orifice to move the first tube relative to the second tube.
The present invention also provides a fuel injector. The fuel injector comprises a body, a closure member, a piezoelectric device, and a compensator. The body extends along an axis. The closure member is displaceable with respect to the body between a first configuration and a second configuration. The first configuration prevents fuel flow through the body, and the second configuration permits fuel flow through the body. The piezoelectric device displaces the closure member from the first configuration to the second configuration. The compensator is coupled with the piezoelectric device and includes a first tube, a second tube, a piston, and fluid. The first tube extends along the axis from a first end portion that occludes the first tube. The first end portion contiguously engages a first one of the body, the closure member, and the piezoelectric device. The second tube is telescopically received in the first tube. The second tube extends along the axis from a second end portion that generally occludes the second tube and that defines an orifice. The second tube is fixed with respect to a second one of the body, the closure member, and the piezoelectric device. The piston is telescopically received in the second tube. And the fluid is displaceable through the orifice to move the first tube relative to the second tube.
The present invention also provides a method of assembling a compensator for a fuel injector. The method comprises providing a first tube extending along an axis from a first end portion occluding the first tube, filing the first tube with a volume of a fluid, inserting a second tube telescopically in the first tube, and inserting a piston telescopically in the second tube, inserting a plug in the piston. The second tube extends along the axis from a second end portion that generally occludes the second tube and that defines an orifice. The orifice is submerged in the volume of the fluid. The piston includes an aperture, and the plug is inserted in the aperture.